1. Field of the Invention
The present invention relates to an endoscopic imaging system and an endoscope system for producing endoscopic images.
2. Description of the Related Art
In recent years, endoscopes have been widely adopted in the fields of medicine and industries. Recently, an endoscopic imaging system having an endoscopic imaging apparatus for producing endoscopic images has come to be used widely. For example, Japanese Unexamined Patent Publication No. 6-86138 has disclosed a related art.
According to the related art, since a signal delay occurs over a cable linking an imaging device and an image processing unit (or video processing unit), a correction needs to be made for cables having different lengths. For this purpose, a reference signal is produced using an output signal of a charge coupled device (CCD). A phase-locked loop (PLL) composed of a phase adjustment circuit, a low-pass filter (LPF), and a voltage-controlled oscillator (VCO) is used to adjust the phases of signals and produce various timing pulses.
A problem with the Japanese Unexamined Patent Publication No. 6-86138 lies in that the PLL is used to adjust the phases of signals and produce various timing pulses in effort to make corrections for cables of different lengths. This makes the circuitry in the image processing unit (or video processing unit) complex. When a plurality of types of electronic endoscopes having cables of different lengths are connected, it is difficult to operate the PLL precisely.
Moreover, a CCD drive circuit is incorporated in the image processing unit (or video processing unit). When solid-state imaging devices that are driven under different conditions are used, drive circuits associated with the solid-state imaging devices must be incorporated in the image processing unit. This results in complex circuitry.
According to related arts disclosed in Japanese Unexamined Patent Publication Nos. 6-105807 and 5-176883, the phases of sampling signals used to perform correlative double sampling are adjusted according to the length of a cable. According to the disclosed methods, the phase of a clock used in a video processing unit for processing a video signal must be adjusted accordingly. Phase difference information stemming from the phase adjustment must be supplied to the video processing unit in any form.
Moreover, the timing of video signal processing must be varied depending on the phase difference information.
Japanese Unexamined Patent Publication No. 63-1263 has disclosed an endoscopic imaging system in which a delay circuit for delaying driving signals used to drive a solid-state imaging device is located around an operation unit of an electronic endoscope.